Sound recording and reproducing apparatus



Jan. 11, 1938. c. M. slNNl-:TT ET Al. 2,105,167

y SOUND RECORDING AND REPRODUCING APPARATUS y Filed Oct. 3l, 1935 N Iv Emi Ze C. Quedan H7' TORNEY Patented Jan. ll, 1938 2,16? SOUND GORDING AND REPRODUCING .rit TU@ chamiu. a.'

Guedon. Audubon, N.

Corporation of America,

Westmont, and Emile G.

.. assignors to Radio a corporation ofy Delaappiicauon october si, i935, semi No. 47,658

This invention relates to sound uur and reproducing apparatus and more particularly to an electro-magnetic phonograph pickup for translating mechanical movements linto electrical, impulses which may subsequently be reproduced by a loud-speaker afterbeing ampliiied by a thermion'ic amplifier.

In. the operation of electromagnetic phonograph pickups from a record mounted on a ttable and driven by an alternating current motor, it has been found that objectionabie has been 'induced in the armature and winding of the pickup from the external stray magnetic'eld of the motor. The disturbing duk iro the motor, and/or, from other electrical power apparatus ordinarily used in connection with phonograph reproduction, has induced hum in the pickup in spite of the use of magnetically peeable objects with shielding functions, such asthe motorboard and turntable, ordinarily interposed between the pickup and'source of disturbance, or Vby the use of a steel plate intended to shield out hum flux.

' Various attempts have heretofore been made by C. M. Sinnett to neutralize humas by the use of an external bucking coil, or pickup coupling transformer, mounted within a phonograph cabinet and properly oriented with respect to a source of disturbance. While suchl a ments gave satisfactory results they had the disadvantage that neutralization was not entirely effective for all positions of the pickup in its movement across the record.

An object of the invention is to provide animproved electromagnetic pickup characterized by means for preventing the reproduction of disturbances or hum, set up in the pickup by magnetic induction, regardless of the noi operating position of the pickup.

A further object of the invention is to provide an electrical pickup with improved neutralizing means in a compact and unitary structure.

More specifically, it is an object, of our invention to provide a simplified, and inexpensive means within a phonograph pickup, for neutralizing induced disturbances from a nearby source of disturbing flux such as adjacent power apparatus, and which is adapted to modern methods of quanl tity production and assembly.

In accordance with our invention we have mounted a neutralizing or hum bucking coil structure, connected in series with a translating or pickup coil, on the pickup assembly for movement with the pickup across a record. More specifically in accordance with our invention the 15 (ci. 11s-1004i) neutralizing coil structure is mounted closely adjacent to the pickup coil and armature structure, within the pickup casing, in such/manner as to insure that the two structures will be subjected substantially in like manner to the disturbing iiux while only the pickup coil has useful sound representing currents induced therein. In furtherance of the desireddisturbance eliminating` eiiect, the neutralizing coil structure is constructed to simulate the` electromagnetic conditions of the pickup coil and amature.

The 'foregoing arrangement possesses several advantages e. g., it produces proper neutralization regardlessof the position of the pickup in its path across the record and it is more effective, simpler and cheaper than mounting the neutralizing coil structure externally of the pickup assembly. Speciiically, mounting the coil within the pickup casing is much preierableto mounting it outside the casing, as on the tone arm.

Other advantages and object will be apparent from a consideration of an embodiment of our invention 4as .described in the present specification and illustrated in the accompanying drawing in Y which;

Figure 1 is'an enlarged front view in elevation, of a phonograph pickup embodying our invention, with some portions removed or broken away,

Fig. 2 is a side view of the pickup illustrated in Fig. l, shown partly in section, as viewed in' the direction of the arrows 2---2 in Fig. 1,

. Fig. 3 is a schematic diagram of coils and connections within the pickup, and

Fig. 4 is a view in perspective of an element oi the assembly shown in Figs. l and 2.

Referring to the drawing, a pickup housing including a base I of non-magnetic material, preferably molded phenolic resin known as bakelite, has mounted thereon a pair of magnetically permeable pole pieces 3 and 5 having pole tips 1 and 9, respectively, and a field magnet il, preferably of the permanent type, as of cobalt steel. The pole pieces are preferably welded to the field magnet at I3, and are thereby rigidly supported with thev tips 'in permanently Aspaced relation to form an active air-gap, in accordance with the teachings of Guedon, Serial No. 34,016, led July 31, 1935. An armature I1 of magneticallypermeable material, provided with stylus receiving means I9, is supported on itsshaft 23 by an armature shaft bearing clamp that bridges the pole pieces. 'Ihe armature at its upper end is iiattened for transverse movement in the air gap in the usual manner.' This clamp consists of a lower bar 2.5 of magnetically permeable mate- 55 of insulation material.

together, and permit lateral adjustment and centering oi the armature by reason of the clearance provided by enlarged holes in the clamp.

A pickup coil 33 surrounds a portion ofthe armature and is carried by the armature clamp. The armature forms with the pole structure in local magnetic circuit for inducing currents in said coil in accordance with the vibrations of the armature. An angular centering or restoring spring 35, is soldered at one end to an end of the armature shaft and is adjustably clamped at the other end by a clamping plate 31 and screw 39, in accordance with Guedon Patent 2,086,415, issued July 6, 1937. A block of damping material 4l, described and claimed in Kendall Patent 2,092,- 884, issued Sept. 14, 1937, is attached at its lower end to the rear end of the armature shaft, and is clamped at its upper end to the molded base by means of a bolt 42 and clamping bracket 43. The bracket at one end is in the form of an angularly bent plate 45 for clamping the damping block, and at its other end is bent and tapped at 41 for receiving a screw which in turn is adapted. to hold the cover plate, not shown, in place on the insulating base.

A neutralizing coil 5I is connected in circuit in series with the pickup coil 33 in opposed phase relation, by means of leads 50, 52, 54, as shown diagrammatically in Fig. 3. It is mounted on a bracket 53 preferably of non-magnetic material, such as brass, by means of a bolt 55 or core of magnetic material. The bracket is shown more clearly in Fig. 4, and consists of a piece of sheet material angularly bent to form a shelf portion 51 and a securing flange 53, each provided with an attachment aperture 6| and 63, respectively. The bracket is conveniently supported as by attachment of its flange portion to the pole piece 3 by means of a screw 55. A rubber washer 56 is employed between the bolt nut and the coil. 'Ihe bolt has a shoulder against which the nut acts as a limit. The bracket shelf has an extension 63 that rests on the top of the pole piece for additional support that is desirable in view of the considerable magnetic attraction between the neutralizing coil structure and the magnetic pole structure.

vThe neutralizing coil is preferably provided with about the same number of turns as the pickup coil and the attachment bolt 55 functions as a'core, generally simulating the magnetic conditions of the armature in the pickup coil. 'I'he neutralizing coil structure is desirably mounted close tothe pickup coil and armature, and coaxially therewith, whereby both coils are subjected substantially in like manner to the disturbing' hum'flux, shown by dotted arrows in Fig. 3, that induces disturbing currents, indicated by full line arrows, in both coils in a substantially equal and opposite sense. Under these conditions the hum is bucked out and no hum current ows in the translating coil circuit.

The'head of the bolt or core of the neutralizing coil is so close to the active air gap of the pole tips that a substantial amount of flux would be short-circuited or by-passed around the gap. We hr ve solved this difficulty by providing a bolt with a head slotted at 61 and arranging for the slot (screw-driver) in the assembly process to be parallel with the air gap. This breaks the bypass action by increasing the reluctance of the shunt path caused by the bolt head. While the neutralizing coil is disposed in close relation to the pickup armature and pole pieces for hum bucking, it is desirable that the coupling coefficient be low from the standpoint of elciency of operation of the pickup. In other words, the neutralizing coil is mounted substantially externally of the local magnetic circuit of the pickup coil, and is substantially independent of the useful magnetic induction by the armature and local magnetic circuit. y

The position of the neutralizing core, particularly its spacing from the pickup armature assembly, may be adjustable if desired, as by bending the bracket or by moving about the pole piece attachment screw 65. It will be noted that the neutralizing coil and the bracket form a subassembly unit for convenient handling and assembly, and may in some cases be added to standard pickups already in use and not provided with hum neutralizing means.

While we have illustrated our invention in connection with a magnetic armature type pickup, it should be obvious to those skilled in the art that it also has utility in connection with a moving coil type pickup wherein a translating coil is caused to vibrate in a magnetic air gap by means of a stylus. It will also be obvious, without additional illustration, that our invention may be applied to a recorder or record cutter. In such case a reverse action occurs, electrical impulses, corresponding to sound, being fed into the device for conversion into a mechanical record.

It will be seen that we have provided a new and useful pickup with a neutralizing coil in a compact, unitary, efficientand inexpensive structure. We do not desire to be restricted to phonograph pickups, since it should be obvious that the invention can be applied to electromagnetic microphones, commonly called pickups, driven directly by sound waves instead of by a record of sound waves. Our invention therefore is not to be limited except insofar as is necessitated by the prior art or bythe spirit of the appended` claims.

We claim as our invention:

1. In a pickup device subjected to a source of disturbing magnetic flux, a translating coil, a housing therefor, means for magnetically inducing useful electrical currents in said coil in accordance with sound waves, a neutralizing coil connected in series opposing relation with said coil, and structure for mounting said neutralizing coilon'said device adjacent to said translating coil, but substantially independent of magnetic induction by said means, whereby both coils are subjected substantially inv like manner to said disturbing flux for balancing out the effects of said flux.

2. The invention as set forth in claim 1 characterized in that said coils are mounted in coaxial relation and within said housing.

3. In an electrical pickup subject to an external field of disturbing ux, a field magnet, pole pieces therefor provided with pole tips spaced apart to form an active air gap, an armature of 105,107 said magnetic circuit and coaxially with respectto said translating coil, whereby both oi' said coils are subjected substantially in like amount to said disturbing flux. A

4. The invention as set forth in claim 3 characterized in that said neutralizing coil is provided with substantially the same number of turns as said translating coil and with a magnetic core for substantially simulating the magnetic conditions of said translating coil whereby eii'ective neutralization -of undesired flux is obtained.

5. In a phonograph pickup subject to an external field of disturbing iiux, afield magnet, pole pieces therefor provided wit jpole tips -spaced apart to forml an active air'gap, an armature of magnetic material vsupported from said pole pieces for movement yin said air gap,l a translating coil disposed around said armature, a neutralizing coil connected in circuit with said translating coil and provided with any open core of magnetic material, said core having one end disposed closely adjacent "to said air gap, and a slot in said' end disposed in a direction parallel with said air gap for minimizing the by-pass action of said core on said air gap.

6. The invention as set f orth in claim 5 characterized in that said core is a bolt constituting an element of support structure for mounting said neutralizing coil-and further that said slot constitutes a screw driver slot in the head of said bolt 7. In a phonograph pickup, a housing, a magnet and pole piece structure supported thereby, said pole pieces having pole tips spaced apart to form avtransverse air gap, an armature of :nag`

. netic material supported for vibration in said air gap, a translating coil disposed around said armature, a neutralizing coil connected in circuit with said translating coil, a bracket within said housing for supporting said neutralizing coil, said bracket consisting of a flanged portion for attachment to one of said pole pieces and a shelf portion for supporting said coil, and a bolt for clamping said coil in place on said shelf portion, said bolt being of magnetic material for forming a magnetic core.

v 8. The invention as set forth in claim 7 characterized in that one end of said bolt is disposed adjacent said air gap and is slotted in a direction parallel to said air gap.

9. In a sound recorder and reproducer adapted to be subjected to a source 'of disturbing flux, a translating coil, a movable core'adapted to vibrate in accordance with sound waves, a neutralizing coil disposed in coaxial relation with respect to said translating coil, a magnetic core for said neutralizing coil disposed in substantial alignment with said other core and having an end disposed in close inductive relation to an end of said movablev core whereby both coils are subjected in like manner. to said flux for balancing out the effect of said fluir.`

10. The invention as set forth in claim 9 characterized by means for minimizing an undesired eifect on the magnitude of useful currents in for opration with a said translating coil caused by the close proximit of said neutralizing coil core.

11. A sub-assembly for an attachment to -a phonograph pickup of the electro-magnetic vibrating armature type comprising a neutralizing coil, a support bracket for said -coil of non-magnetic material, a bolt of magnetic material disposed axially within said coil for clamping said coil to said bracket, and Ameans for'securing said bracketv to a pole piece structure of said pickup formounting the coil relation to the pickup.

12. A sub-assembly for an attachment to a phonograph pickup of the electro-magnetic vibrating armature type comprising a neutralizing coil, a support bracket of non-magnetic material 10 and core in close inductive y having shelf and an angularly disposed iianged portion, a bolt of magnetic material disposed axially within said coil and through an aperture in said shelf portion for clamping said. coil tsaid bracket, and an aperture in said flanged portion adapted to b e attached to a pole piece structure of said pickup for mounting the coil and core in close inductive relation to the pickup.

13. In combination, an electrical pickup device for operation with a sound record, an external source of electrical disturbance affecting said device, means for mountingsaid device for movement across said record, neutralizing means electrically connected with said device and mounted on said device in fixed relation thereto for similar movement therewith, whereby neutralization of said disturbance is obtained regardless of the operating position of said device across said record.

14. In combination, an electrical pickup device sound record, a, source of disturbing magnetic ux, a translating coil, means for magnetically inducing useful electrical currents in said coil in accordance .with sound waves. a neutralizing coil connected in series opposing relation with said translating coil, and structure for mounting said neutralizing coil on said device, substantially independent of said useful magnetic induction, whereby both coils are subjected substantially in like manner to said disturbing ux irrespective of the position of said device on said record. y

15. In combination, a phonograph pickup, a nearby source of disturbing magnetic flux, a translating coil, a vibratory armature operatively disposed with respect to said coil, magnetic pole structure forming with said armature a local magnetic circuit for inducing currents in said coil in accordance with the vibrations of said armature, a neutralizing coil connected with said translating coil, and means for mounting said neutralizing coil substantially externally of said local magnetic circuit but in close physical rela- 

